1. Field of the Invention
The present invention relates to a thermal transfer recording medium, such as color thermal transfer paper or a color thermal transfer ribbon, which is capable of reproducing gradation of, for example, photographs as it is used in a thermal recording system by means of a thermal recording member, such as a thermal head or a thermal pen, and also relates to an image forming body (on which an image is printed) suitable for use in the thermal transfer recording system.
2. Description of the Prior Art
According to one method of reproducing, in printed form, image data contained in, for example, a video signal, by means of a thermal recording device such as a thermal head or a thermal pen, a thermal transfer recording medium obtained by forming a transfer layer containing a heat sublimation transfer disperse dye or a hot-melt evaporation transfer dye on a substrate sheet is superposed on a transfer medium, thereby transferring the image information.
A butyral resin is mainly used as a binder for the transfer layer.
In order to perform clear printing based on a video signal or the like in accordance with the above method, a thermal transfer recording medium must smoothly form from a high density portion to a low density portion in correspondence to energy applied by a thermal head. However, if high energy is applied from the thermal head, not only is an image transferred to an image forming body but also the binder contained in the transfer layer of the transfer recording medium. On the other hand, if a low energy is applied, transfer by sublimation or evaporation is not sufficiently performed. Therefore, it is difficult to transfer a dye with gradation in correspondence to an image signal.
According another image transfer method, a thermal transfer sheet including a sublimation disperse dye layer in which an amount of a dye to be transferred can be changed in accordance with a heat amount of a thermal head is used to control an amount of a sublimation dye, thereby obtaining a photographic image with gradation (Image Electronics Society, Vol. 12, No. 1, 1983).
As a technique similar to the above method, dry transfer printing with respect to a polyester fiber is already known. According to this method, a dye such as a sublimation disperse dye is dispersed or dissolved in a synthetic resin solution, so as to obtain a color material, and the color material is printed in a pattern on thin paper and dried to prepare a transfer sheet. Then, the transfer sheet is superposed on a cloth formed of polyester fibers and transferred with heat and pressure to transfer the dye into the polyester fibers, thereby obtaining an image. However, it is difficult to obtain an image having high density even if printing is performed by a thermal head using this transfer paper. The reasons for this are as follows:
The dye used in this transfer paper can be transferred upon heating by a hot plate at 200.degree. C. for about 60 seconds. Therefore, since a heat amount is small, i.e., applied from the thermal head at about 250.degree. C. for only several milliseconds (n/1000 sec; n=an integer below 10), transfer is not sufficiently performed. Moreover, a thickness of the thin paper is as small as at most 10 .mu.m. Therefore, it is difficult to increase a temperature of the transfer layer in a short period of time up to a temperature at which a heat amount is absorbed in the paper to sublimate and transfer the dye.
Various improvements in which, e.g., a film is formed very thin (6 to 9 .mu.m) have been proposed although they are basically the same as the above principle.
Examples are Japanese Patent Disclosure (Kokai) Nos. 60-994 and 60-101087. In these methods, however, it is difficult to obtain an image having high density. Meanwhile, if a high energy is applied to obtain an image having high density, a printing speed is reduced to reduce a service life of a thermal head.
Another problem of the thermal recording medium having the thermal transfer layer in which the dye is sublimited or melted to be transferred with heat is a phenomenon in which a resin constituting the transfer layer of the recording medium partially is transferred to a transfer medium, i.e., an image forming body during transfer. When not only the dye but also the resin are transferred during transfer recording, density control of a transferred image becomes insufficient, and therefore gradation cannot be accurately formed. In order to eliminate such a drawback, Japanese Patent Disclosure (Kokai) Nos. 59-14994, 59-71898, 61-189994, and 61-188193 propose methods in which a polyamide resin is used as a binder resin constituting the transfer layer. According to these methods, adhesion of the transfer layer to a supporting body is more or less improved to reduce migration of the resin to the image forming body. However, during storage, the transfer layer easily absorbs humidity, the dye in the transfer layer tends to be transferred, or the dye tends to be transferred to a material superposed thereon even while heat is not applied. Therefore, these methods are not sufficient in terms of storage stability.
Meanwhile, an image forming body for forming a transfer image from a thermal transfer medium containing a sublimation dye in a thermal transfer layer must have good dyeing property, light resistance, chemical resistance, and abrasion resistance, and the thermal transfer medium and the image forming body must have a blocking (resin film peeling) preventing property during printing. Furthermore, in recent years, in order to form an image on the basis of information of, e.g., a video signal, a thermal transfer medium using a sublimation dye has been increasingly used. Therefore, a demand has arisen for an improvement in storage stability, especially, a plasticizer resistance or a retransfer resistance of the formed image.
In order to transfer the sublimation dye of the thermal transfer medium to the image forming body well to form colors, a resin at the image forming body side must have a good dyeing property with respect to the sublimation dye. The dyeing property of the resin is better when a softening point and a glass transition point are lower. However, the resin having the good dyeing property tends to be melted to cause blocking with a resin which holds the sublimation dye of the thermal transfer medium during thermal transfer. In addition, the sublimation dye once dyed tends to be sublimated again to degrade image quality. A blocking phenomenon in thermal transfer is also associated with a resin coating amount with respect to the transfer medium. Therefore, a certain countermeasure must be performed to the resin layer of the image forming body to prevent blocking especially from the dye transfer medium side to the image forming body side. For example, the surface resin layer of the image forming body may be hardened to obtain a heat resistance. In this case, however, it is difficult to transfer the sublimation dye of the transfer medium to the image forming body, thereby degrading image density.
In addition, an additive such as a silicone oil may be mixed in the resin layer of the image forming body, or the silicone oil or the like may be coated on the resin surface of the image forming body to activate the image forming body surface. In this case, however, the image forming body surface becomes viscous and dirt tends to be adhered thereon, thereby posing a problem of finger print resistance. As described above, a good dyeing property of the sublimation dye from the transfer medium to the image forming body is in a reciprocal relationship with the storage stability after dyeing, and no countermeasure is proposed so far.